1. Field of the Invention
The invention relates to an electric storage device.
2. Description of the Related Art
A secondary battery is widely used as a power source for a portable electronic device such as a laptop computer, or a cellular phone. The secondary battery is also used as a power source that provides power for an electric vehicle and the like. To suppress deterioration of the performance of the secondary battery, various proposals are made.
Japanese Patent Application Publication No. 2005-11660 (JP-A-2005-11660) describes an electrode for a secondary battery. In the electrode for a secondary battery, plural small cells, which serve as electrode layers, are formed on a current collector, which serves as a substrate, to reduce thermal stress due to a change in the temperature in the secondary battery.
The publication No. 2005-11660 (for example, FIGS. 1, 2, and 11) describes that the amount of a conductive agent varies among the plural small cells formed on the current collector so that temperature is uniform in the electrode for a secondary battery. More specifically, the amount of the conductive agent contained in the small cell positioned in the center portion of the current collector is smallest, and the amount of the conductive agent contained in the small cell positioned in the end portion of the current collector is largest.
However, in the electrode for a secondary battery described in the publication No. 2005-11660, there are regions where the small cells are formed, and regions where the small cells are not formed, on the surface of the current collector. With this configuration, because electric current does not flow in the regions where the small cells are not formed, the energy efficiency of the secondary battery decreases.
Also, the area of each small cell is extremely small as compared to the area of the current collector. Therefore, the small cells are likely to separate from the current collector, for example, in a process of manufacturing the electrode for a secondary battery, or in a process of disposing the secondary battery that includes the electrode for a secondary battery. When the plural small cells are formed on the current collector, the electrode for a secondary battery is easily bent. However, the small cells are likely to separate from the current collector due to stress when the electrode for a secondary battery is bent.
In the configuration where the amount of the conductive agent varies among the small cells so that the resistance value varies among the small cells, the resistance value is uniform in each small cell. Therefore, when the temperature varies in the region where each small cell is formed, it is not possible to suppress the variation in the temperature in the region.
When the amount of the conductive agent varies among the small cells, and the amount of an active material is substantially uniform in all the small cells, part of the active material is not used for charging/discharging of the secondary battery. That is, in the small cell that contains a small amount of the conductive agent, all the active material contained in the small cell may not be used for charging/discharging of the secondary battery. The raw material of the active material is relatively expensive. Therefore, if part of the active material is not used for charging/discharging of the secondary battery, the cost of the electrode for a secondary battery is increased.